In general, the compression ratio of an internal combustion engine is represented by the largest volume of a combustion chamber prior to compression and the smallest volume of the combustion chamber after compression during a compression stroke of the internal combustion engine.
The output of the internal combustion engine increases as the compression ratio of the internal combustion engine increases. However, if the compression ratio of the internal combustion engine is too high, such that knocking occurs, the output of the internal combustion engine decreases, and overheating of the internal combustion engine and a failure in a valve or piston of the internal combustion engine and the like occur.
Accordingly, the compression ratio of the internal combustion engine is set to a specific value within an appropriate range prior to the occurrence of knocking. As such, because the air-fuel ratio and output of the internal combustion engine can be improved by properly varying the compression ratio according to the load of the internal combustion engine, various approaches are being proposed to vary the compression ratio of the internal combustion engine.
These approaches for varying the compression ratio of the internal combustion engine in general employ methods that vary the volume of the compression chamber during the compression stroke.
For example, there have been proposed methods that vary the height of the top dead center of a piston during a compression stroke, or increase or decrease the volume of a sub-compression chamber provided in a cylinder head.
Varying the height of the top dead center of the piston tends to make the structure of the internal combustion engine complicated. Therefore, it may be desirable to vary the compression ratio by providing a sub-compression chamber in the cylinder head to make the structure simple and achieve great improvement in air-fuel ratio.
However, since combustion impact in a combustion stroke is directly transmitted to elements of a variable compression ratio device, durability of the elements may be deteriorated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.